Here are 20 of the best SEO plugins to help you choose the right tags, tell search robots what to work on, optimize your post titles and more.

All in One SEO Pack – One of the most popular plugins ever for WordPress, this plugin does a bit of everything for you from helping choose the best post title and keywords, to helping you avoid duplicate content and more.

Automatic SEO Links – Automatic SEO Links allows you to choose a word or phrase for automatic linking, both internal and external, set anchor text, choose if it should be “nofollow” or not, and more. One of the best features of this plugin is that it will only do this for the first occurrence of a word in a post so you don’t have to worry about spamming your post with numerous links to the same thing.

Google XML Sitemaps – An essential tool in any blogger’s armory of SEO tools. While the name only mentions “Google,” this plugin creates an XML-sitemap that can be read by Ask, MSN and Yahoo also.

HeadSpace2 – This plugin allows you to install all sorts of meta-data, add specific JavaScript and CSS to pages, suggests tags for your posts and a whole lot more.

Meta Robots WordPress plugin – An easy solution for adding robot metadata to any page you choose on your blog. You can use it to make your front page links into “nofollows,” prevent indexing of search pages, disable author and date-based archives, prevent indexing of your login page and numerous other features.

Nofollow Case by Case – This plugin allows you to strip the “nofollow” command from your comments, and then you can apply it to only the comments you don’t wish to support.

Platinum SEO Plugin – The Platinum SEO Plugin offers you such features as automatic 301 redirects for permalink changes, auto-generation of META tags, post slug optimization, help in avoiding duplicate content and a host of other features.

Redirection – For any number of reasons you sometimes need to move a page from one spot on your blog to another, but then you risk losing that page’s status in search results. Redirection helps you with your 301 redirects, captures a log of 404s so you can work on correcting them, sets up an RSS feed for errors and more.

SEO Blogroll – Do you worry that the people you link to in your blogroll are feeding off of your PageRank? With SEO Blogroll you can make separate sections for various groupings of links, with an unlimited number in each, and all of them will receive the “nofollow” attribute.

SEO for Paged Comments – With the introduction of paged comments in WordPress 2.7, there was a potential problem with search engines thinking you had duplicate content as the post would appear on each page. This plugin aims to take care of this issue for you until the folks at WordPress change things up.

SEO friendly and HTML valid subheadings – Some themes for WordPress will confuse your sub-header tags based on the page they are to be displayed on, but this plugin will automatically reset them to make them more SEO friendly by moving them down one spot in the hierarchical tree. In other words, h2 becomes h3, h3 becomes h4 and so on.

SEO Friendly Images – Images can be a great source of traffic as people search for images of various subjects, and this plugin helps you with making sure that you have “alt” and “title” tags on all of your images so that the search engines can properly index them.

SEO No Duplicate WordPress Plugin – If you must have duplicate content on your site for whatever reason, SEO No Duplicate will allow you to state which version of the post search engines should index while ignoring the others.

SEO Post Link – The post slug is the blog title you see in a browser’s URL bar, and if it’s too long, search engines won’t take a liking to it. SEO Post Link comes with an already populated list of words to cut from a title when it turns into a URL to make your post addresses that much friendlier. You can set it so that it’s limited to a certain number of characters, cut short words, cut unnecessary words and more.

SEO Smart Links – Interlinking your blog can be the key to getting more people to read more of your posts, but it is time consuming and tedious to do it by hand. SEO Smart Links does this for you automatically when you tell it what words to link to what URLs, and it also allows you to set “nofollow” and “open in window” comands for the links.

SEO Tag Cloud Widget – Love ‘em or hate ‘em, a lot of people use tag clouds on their blogs. Since their inception they have been fairly unreadable by search engines, but with this plugin they will be converted to an SEO-friendly HTML markup that can be indexed.

SEO Title Tag – Your tags are an important part of your site for making sure that search engines know where to place your posts, and SEO Title Tag focuses exclusively on this. Unlike some other plugins, and WordPress itself, this extension will allow you to add tags to your pages, your main page and even any URL anywhere on your site.

Simple Tags – An extremely popular plugin that focuses on helping you choose the best tags for your posts by offering suggestions, auto-completion of tags as you type, an AJAX admin interface, mass tag editing and a whole lot more.

Sitemap Generator – This is a more customizable sitemap generator than most with options to support multi-level categories and pages, category/page exclusion, permalink support, choices on what to display, options to show number of comments and more.

TGFI.net SEO WordPress Plugin – This particular plugin will do most of the usual SEO work of optimizing titles and keywords, but it adds a unique twist as it is mainly directed at people who use WordPress as a CMS.